I'm doing a large print, and about 1/4 of the way through, I started hearing this horrible, intermittent noise. Sounds very close to nails on a chalkboard. Naturally I assume something is starting to wear out (belt, roller, motor, etc) or there's some parts rubbing together that shouldn't.
The print seems to be going along fine, though, so I let it be until I couldn't stand it anymore.
Finally sat down and watched it, and the culprit is the extruder fan blowing into the large, hollow tree support bases which was acting like a whistle. lol.
I've only been 3D printing for a little over a year, and this is the largest print I've ever done so far, so if this is normal, it caught me totally off guard.
This is approximately where it is in the print. Every time the head moves over any of those support bases, it sounds like nails on a chalkboard 😆